What is the Python equivalent of static variables inside a function?

Question

What is the idiomatic Python equivalent of this C/C++ code?

void foo()
{
    static int counter = 0;
    counter++;
          


        

Answer 1

I personally prefer the following to decorators. To each their own.

def staticize(name, factory):
    """Makes a pseudo-static variable in calling function.

    If name `name` exists in calling function, return it. 
    Otherwise, saves return value of `factory()` in 
    name `name` of calling function and return it.

    :param name: name to use to store static object 
    in calling function
    :type name: String
    :param factory: used to initialize name `name` 
    in calling function
    :type factory: function
    :rtype: `type(factory())`

    >>> def steveholt(z):
    ...     a = staticize('a', list)
    ...     a.append(z)
    >>> steveholt.a
    Traceback (most recent call last):
    ...
    AttributeError: 'function' object has no attribute 'a'
    >>> steveholt(1)
    >>> steveholt.a
    [1]
    >>> steveholt('a')
    >>> steveholt.a
    [1, 'a']
    >>> steveholt.a = []
    >>> steveholt.a
    []
    >>> steveholt('zzz')
    >>> steveholt.a
    ['zzz']

    """
    from inspect import stack
    # get scope enclosing calling function
    calling_fn_scope = stack()[2][0]
    # get calling function
    calling_fn_name = stack()[1][3]
    calling_fn = calling_fn_scope.f_locals[calling_fn_name]
    if not hasattr(calling_fn, name):
        setattr(calling_fn, name, factory())
    return getattr(calling_fn, name)

Answer 2

Sure this is an old question but I think I might provide some update.

It seems that the performance argument is obsolete. The same test suite appears to give similar results for siInt_try and isInt_re2. Of course results vary, but this is one session on my computer with python 3.4.4 on kernel 4.3.01 with Xeon W3550. I have run it several times and the results seem to be similar. I moved the global regex into function static, but the performance difference is negligible.

isInt_try: 0.3690
isInt_str: 0.3981
isInt_re: 0.5870
isInt_re2: 0.3632

With performance issue out of the way, it seems that try/catch would produce the most future- and cornercase- proof code so maybe just wrap it in function

Answer 3

This answer builds on @claudiu 's answer.

I found that my code was getting less clear when I always had to prepend the function name, whenever I intend to access a static variable.

Namely, in my function code I would prefer to write:

print(statics.foo)

instead of

print(my_function_name.foo)

So, my solution is to :

1. add a statics attribute to the function
2. in the function scope, add a local variable statics as an alias to my_function.statics

from bunch import *

def static_vars(**kwargs):
    def decorate(func):
        statics = Bunch(**kwargs)
        setattr(func, "statics", statics)
        return func
    return decorate

@static_vars(name = "Martin")
def my_function():
    statics = my_function.statics
    print("Hello, {0}".format(statics.name))

---

Remark

My method uses a class named Bunch, which is a dictionary that supports attribute-style access, a la JavaScript (see the original article about it, around 2000)

It can be installed via pip install bunch

It can also be hand-written like so:

class Bunch(dict):
    def __init__(self, **kw):
        dict.__init__(self,kw)
        self.__dict__ = self

Answer 4

Building on Daniel's answer (additions):

class Foo(object): 
    counter = 0  

def __call__(self, inc_value=0):
    Foo.counter += inc_value
    return Foo.counter

foo = Foo()

def use_foo(x,y):
    if(x==5):
        foo(2)
    elif(y==7):
        foo(3)
    if(foo() == 10):
        print("yello")


use_foo(5,1)
use_foo(5,1)
use_foo(1,7)
use_foo(1,7)
use_foo(1,1)

The reason why I wanted to add this part is , static variables are used not only for incrementing by some value, but also check if the static var is equal to some value, as a real life example.

The static variable is still protected and used only within the scope of the function use_foo()

In this example, call to foo() functions exactly as(with respect to the corresponding c++ equivalent) :

stat_c +=9; // in c++
foo(9)  #python equiv

if(stat_c==10){ //do something}  // c++

if(foo() == 10):      # python equiv
  #add code here      # python equiv       

Output :
yello
yello

if class Foo is defined restrictively as a singleton class, that would be ideal. This would make it more pythonic.

Answer 5

A bit reversed, but this should work:

def foo():
    foo.counter += 1
    print "Counter is %d" % foo.counter
foo.counter = 0

If you want the counter initialization code at the top instead of the bottom, you can create a decorator:

def static_vars(**kwargs):
    def decorate(func):
        for k in kwargs:
            setattr(func, k, kwargs[k])
        return func
    return decorate

Then use the code like this:

@static_vars(counter=0)
def foo():
    foo.counter += 1
    print "Counter is %d" % foo.counter

It'll still require you to use the foo. prefix, unfortunately.

^{(Credit: @ony)}

Answer 6

Soulution n +=1

def foo():
  foo.__dict__.setdefault('count', 0)
  foo.count += 1
  return foo.count